Carter Page, a former investment banker and foreign policy adviser for Donald Trump's presidential campaign, is at the center of another partisan fight.

Documents released on Saturday detail the FBI's Foreign Intelligence Surveillance Court (FISA) applications to surveil him in order to investigate whether Page was working with Russia to meddle in the 2016 presidential election.

President Donald Trump has seized on the documents to claim his campaign was illegally spied on. Democrats, and some Congressional Republicans, have said they show investigators did nothing wrong, and actually show that Page was an "agent of a foreign power", as the documents allege.

Page was previously at the center of an intense debate over a classified memo about the FISA applications that Republicans said exposed corruption at the highest levels of the FBI and the Department of Justice (DOJ).

The memo accused top officials at the FBI and DOJ of misleading federal judges in seeking the warrants needed to extend secret surveillance of Page during and after the 2016 presidential election.

Here's what you should know about Carter Page:

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Carter Page was born in Minneapolis, but raised in Poughkeepsie, New York.

In 1993, Page graduated from the US Naval Academy. He served in the Navy for five years before working on arms control at the Pentagon and moving to New York for a fellowship at the Council on Foreign Relations.

Security guards stand at the entrance to U.S. Naval Academy in Annapolis, Maryland, in 2007.Susan Walsh/AP

While Page has touted his credentials as an expert on the Russian energy market, a handful of other experts in the field told Politico they didn't even know who he was.

"Strangely I've never heard of Carter Page until this Trump connection," Bill Browder, an American investor who spent years working in Russia, told Politico. "It's odd because I've heard of every other financier who was a player in Moscow at the time."

Over the course of his career in foreign policy, Page has not shied away from criticizing what he has described as US hypocrisy toward Russia. According to the Washington Post, Page has praised Putin as a better leader than former President Barack Obama.

"Washington and other Western powers have impeded potential progress through their often hypocritical focus on ideas such as democratization, inequality, corruption and regime change,” Page said during a speech in Moscow in front of prominent Russian government officials.

Russian state TV anchorman Dmitry Kiselyov listens to Page during a presentation in Moscow on December 12, 2016.Sergei Karpukhin/Reuters

Over the years, Page's dealings in Russia caught the attention of US government officials, who suspected that he may be working as an agent of Moscow. Intelligence officials knew of him as far back as 2013, well before he joined the Trump campaign.

But controversy ensued over Page's role in the campaign when the public learned the FBI had launched a probe into whether Trump or his associates were colluding with Russia to influence the election.

In November 2017, Page testified before the House Intelligence Committee. He confirmed meeting Russian government officials on a trip to Moscow during the US presidential campaign. Page denied any wrongdoing or improper dealings with Russia.

In January, Page became the subject of controversy when Republicans alleged in a memo that officials at the FBI and the DOJ misled a federal judge who authorized surveillance on Page before and after the 2016 campaign.

In March, the Department of Justice's Office of the Inspector General announced that it would review the FBI's and DOJ's application to monitor Page during the election.

Over 400 pages of documents released on Saturday show the FBI's applications to surveil Page, and Department of Justice officials plus judges signing off on those requests. The FBI said it had reason to believe Page was "an agent of a foreign power" and "the subject of targeted recruitment by the Russian government."

On Sunday, Page downplayed his Russian connections. "I've never been an agent of a foreign power by any stretch of the imagination," he said.

Other lawmakers drew different conclusions from the documents. Republican Sen. Marco Rubio said they showed the FBI had justified its reasons to surveil Page. "I don't think they did anything wrong," Rubio said, defending investigators. "There was a lot of reasons unrelated to the dossier why they wanted to look at Carter Page."

Sen. Marco Rubio of Florida appears on CNN's "State of the Union" on July 22, 2018.
Screenshot via CNN